The invention concerns an automatic control system for the transfer of the drive power between a tractor and a trailer.
High requirements are imposed on the systems for the transmission of drive power of vehicles intended for operation in severe cross-country conditions. The basic starting point is that, in a tractor-trailer combination that hauls heavy loads of timber, both the tractor and the trailer are provided with drive gears of their own. Thereat, in particular when there are hills in the terrain, a problem arises concerning how to made the drive power transmission systems of the tractor and of the trailer operate interdependently. When the drive power is lowered in the tractor, the drive power must also be lowered in the trailer. When this doesn't happen, the trailer pushes and, in the case of a combination provided with an articulated joint, there is a risk of the combination becoming folded at the middle. In a corresponding way, when the power requirement becomes higher in the tractor, the drive power must also increase in the trailer.
In Finnish Pat. Appl. No. 863464, a system for the control of the drive power transmission is disclosed, wherein the system consists of two circuits that are closed. This arrangement of equipment requires separate means that produce a signal for automatic effecting of the control. In Danish Pat. No. 1150/80, a system is disclosed wherein the tractor and the trailer do not have separate hydraulic circuits. The trailer employs the hydraulic system of the tractor, and the invention is based on a hydraulic clutch between the hydraulic motor and the drive wheels and on the control of said clutch. On the other hand, in British Pat. No. 1,260,438 a hydraulic circuit is disclosed wherein the control of this trailer drive takes place mechanically depending on the difference in speed between the tractor and the trailer. In the Swedish Pat. No. 396,323, a power transmission solution for a trailer is disclosed wherein the trailer is provided with a combustion engine of its own.